1. Field of the Invention
The present invention relates to artificial kidneys, and more particularly, to a method and system for pumping two liquids in equal quantities and with a continuous flow rate in two separate conduits of an artificial kidney.
2. Description of the Related Art
Artificial kidneys typically pump two liquids, for example, fresh and used dialysis liquid, into and out of a hemodialyser. An example of a known apparatus for pumping the two liquids is illustrated in FIG. 1 and includes, a pumping device 106 having two complementary chambers 112 and 113 associated with four valves 126, 127, 128 and 129, disposed, respectively, in lines 118, 119, 121 and 120. The operation of such a device is effected over two periods. During the first period, the valves 126 and 127 are closed, and valves 128 and 129 are open. A reciprocating piston 109, separating the two complementary chambers 112 and 113, is moved in direction A so as to aspirate a first liquid into chamber 112, and, at the same time, discharge an equal quantity of a second liquid from chamber 113. In the second period, valves 128 and 129 are closed, and valves 126 and 127 are open, and piston 109 is moved in direction R to discharge from chamber 112 a quantity of the first liquid into line 118, and to simultaneously aspirate a quantity of the second liquid into chamber 113 from line 119.
With the device depicted in FIG. 1, it is possible to pump two liquids in substantially equal quantities in two separate conduits, but the flows in the four lines 118, 119, 120 and 121, are discontinous due to the alternating movement of piston 109 during reciprocation.
To mitigate this drawback, apparatus have been proposed which, effectively, comprise two of the devices of FIG. 1 arranged in parallel. Such a system is illustrated in FIG. 2 and includes two pumping devices 206 and 206' having respective complementary chambers 212, 213 and 212', 213' associated with eight valves 226, 226'; 227, 227'; 228, 228'; and 229, 229', disposed in respective conduits or lines connected to the chambers of pumping devices 206 and 206' as shown.
Operation of the device of FIG. 2 is as follows. In a first period, valves 226, 227, 228' and 229' are closed; and valves 228, 229, 226' and 227' are opened. A pair of reciprocating pistons 209 and 209', disposed in pumps 206 and 206', respectively, are then moved in direction A. Movement of piston 209 in direction A aspirates the first liquid into chamber 212 from an intake line 220, and discharges the second liquid from chamber 213 into an outlet line 221. Simultaneously, movement of piston 209' in direction A, discharges the first liquid from chamber 213' into an outlet line 218, and aspirates the second liquid into chamber 212' from an intake line 219.
In a second period, valves 226, 227, 228' and 229' are opened, and valves 228, 229, 226' and 227' are closed. Reciprocating pistons 209 and 209' are then moved in direction R. Piston 209' upon being moved in direction R, aspirates the first liquid into chamber 213', and discharges the second liquid from chamber 212' into line 221. Simultaneously, movement of the piston 209 in direction R aspirates the second liquid into chamber 213 from line 219, and discharges the first liquid from chamber 212 into line 218.
By means of systems such as illustrated in FIG. 2, substantially continuous flow through lines 218, 219, 220, 221 can be attained.
A system similar to the one described above with reference to FIG. 2 is described in German Patent Application DOS 2 634 238. However, the embodiments of the device disclosed in that German patent utilize elastic members disposed in respective chambers rather than reciprocating pistons. The circulation of the fresh and used dialysis liquid comprising the first and second liquids is accomplished by two pumps displacing the membranes in the chambers to displace the dialysis liquid.
In principle, these prior art systems allow two liquids in two separate conduits to be pumped in substantially equal quantities and with a substantially continuous flow. However, small errors occur each time the valves (226, 227, 228, 229, 226', 227', 228', 229') are opened or closed. With a high frequency of opening and closing of the valves, the accumulation of these small errors leads to an even higher overall error.
It is therefore an object of the present invention to provide a method and an apparatus for pumping equal quantities of two liquids through two separate conduits in which the errors associated with the valves opening and closing are substantially reduced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.